1. Technical Field
This disclosure relates generally to an improved structure of a spark plug which is used in an internal combustion engine and a production method thereof.
2. Background Art
Spark plugs are known which produces a sequence of electric sparks to ignite an air/fuel mixture introduced into an internal combustion engine for automotive vehicles or motorcycles. Typical spark plugs are equipped with a metal shell (also called a metal case) to be mounted in the internal combustion engine, a ground electrode joined to the metal shell, and a center electrode working to develop the spark between itself and the ground electrode.
Japanese Patent First Publication No. 2009-54574 teaches plastically forming or pressing the ground electrode to form a protrusion which extends toward the center electrode. The protrusion results in a decrease in area of contact the ground electrode with the flame kernel, as produced by the spark between the ground electrode and the center electrode, thus decreasing a quenching (cooling) effect which takes away the heat of the flame kernel to improve the ignitability of the air-fuel mixture. The protrusion also results in an increase in intensity of magnetic field between the protrusion of the ground electrode and the center electrode, which further enhances the ignitability of the air/fuel mixture.
The surface of the metal shell of the spark plug is sometimes plated for rust or corrosion control. When the ground electrode is also plated and then plastically deformed or pressed to form the protrusion, as in the above publication, it may lead to a concern about the peeling of the plated layer from the ground electrode. The use of such a spark plug may result in a misfire in the engine. In order to avoid this problem, the surface of the ground electrode joined to the metal shell may be masked when the metal shell is plated, after which the ground electrode is pressed to make the protrusion and then plated. This, however, results in an increased complexity of making the spark plug and a decrease in productivity thereof.